Larry Niven and Jerry Pournelle have produced a number of very sucessful collaboration novels. Lucifer's Hammer was one of the first science fiction stories to deal with what would happen if a large mass (like a comet) fell upon the Earth in modern times. Footfall is a fine tale about alien viewpoint and the kinetic energy of very large masses moving at planetary speeds. The Mote in God's Eye is my favorite; great characters and a truly original and absorbing plot make it a great story.